1. Field of the Invention
This invention relates to perforated filter tubes, and more particularly to a lockseamed center tube having a plurality of circumferential beads.
2. Background Information
A wide variety of perforated tubes for use in fuel, liquid, oil or hydraulic filters have long been known in the art. These tubes are typically fabricated from perforated metallic sheet or coil stock cut to predetermined size, formed into a cylinder and then seamed along adjoining edges. These tubes must have adequate structural integrity to withstand relatively high collapse pressures exerted on the tube by fuel, oil, or hydraulic fluid during conventional filtration operations. The thickness or gauge of the sheet or coil stock and the integrity of seam play important roles in providing the tube with the requisite resistance to the collapse pressures.
One common approach to tube fabrication is to form a cylinder from smooth sheet steel of relatively heavy gauge, and provide a lockseam in which the adjoining edges of the sheet are mechanically folded over one another. The lockseam thus includes several thicknesses of material and is therefore thicker than the remaining circumference of the tube. The resulting center tube advantageously includes a structurally rigid tube wall, with the lockseam being sufficiently rigid for filtration operations without the need for additional welding or brazing operations for savings in labor and capital equipment costs relative to welded seams.
A drawback of this approach, however, is that the relatively heavy gauge sheet stock represents a large percentage of the material expense of the tube. It is thus desirable to utilize lighter gauge sheet stock, while still providing the tube with the requisite structural integrity.
One attempt to overcome this drawback has been to use lighter gauge sheet stock and corrugate it to improve the sheet's structural rigidity. The corrugations are generally applied to the sheet prior to bending into tubular configuration. The corrugations are generally terminated prior to reaching the edges of the sheet, so as not to interfere with fabrication of the lockseam. While corrugations tend to improve the rigidity of the tube, they must be kept sufficiently shallow and curvilinear in profile so as to facilitate tube formation without generating buckling, etc. This aspect thus tends to substantially limit the degree of wall rigidity imparted by such corrugations.
An alternative approach is to apply corrugations after tube fabrication. Such post-fabrication corrugating operations have been utilized in the metal can industry and typically include application of pressure to the wall of a can as it rotates about its axis to provide an indentation about the entire circumference thereof. This approach may enable deeper corrugations for improved strength relative to the aforementioned pre-fabrication corrugations. While this approach may operate successfully for relatively light-duty can making operations, this technique, as well as the conventional equipment utilized for this purpose, has generally been inadequate for use in heavier duty applications such as filter tube fabrication involving simultaneous formation of a plurality of corrugations, or fabrication of deeper, generally rectilinear beads. In these instances, the unsoldered, multi-thickness lockseam of the filter tube tends to disengage or pop open.
One approach to ameliorating this popping, may be to weld or solder the lockseam prior to beading, as has been done in the can making industry. However, welding or soldering represents an additional manufacturing step which tends to add undesirable time and expense to the manufacture of the tube.
This and other techniques utilized in the canning industry are thus generally inapplicable to the filtration industry, due to the disparate structural requirements of these two applications. For example, general purpose metal cans such as those used for packaging food or liquids are typically designed to withstand significantly lower pressures than those experienced by a filter tube of similar size. Moreover, the types of pressures are also distinct. Many metal cans are designed to withstand internal or burst pressures, while filter tubes must withstand external or collapse pressures.
In this regard, corrugations or ribbing applied to general purpose metal cans such as used for food, are typically designed to facilitate handling, i.e. to provide texture to the surface of the can to help prevent slippage, and to help the cans resist becoming dented, etc. Contrariwise, deeper corrugations or beads are desired for filter tubes to add substantial rigidity to enable use of lighter gauge sheet stock.
A need thus exists for a filter center tube and method of fabrication thereof, in which a multi-thickness lockseam is capable of being beaded to enable use of relatively light gauge sheet stock.